I am away from my development environment, but Weaver Show Posts was updated fairly recently and is compatible with the latest versions of PHP. (It is also backward compatible). So I’m not sure how your host said Show Posts was not up to date.
If Show Posts is deactivated, then the message can not be responsible for the JSON error. When you get an error like that, WP and PHP will normally write to an error file. I don’t remember the exact changes, but you should be able to look at error logs, or have whatever is causing the error display a message.
Sorry I don’t remember the details of that, but it would be more helpful.